One known type of an electrically driven power steering apparatus of a vehicle is a ball screw nut into which a rack shaft of a rack-and-pinion steering apparatus is inserted, is rotated by an electric motor coaxial with the rack shaft, and a rotational output thereof is converted into a thrust in the longitudinal direction of the rack shaft through a ball screw mechanism.
In this type of electrically driven power steering apparatus, the ball screw nut is so supported as to be rotatable with respect to a housing, which involves the use of bearings. These bearings are disposed at both side ends of the ball screw nut and provided with presser members for preventing the bearings from coming off the ball screw nut.
The presser member is formed with a female thread meshing with a male thread formed on an outer periphery of the ball screw nut. The presser member is thus screwed to the ball screw nut and thus prevents the bearing from coming off.
By the way, for example, in a general type of electrically driven power steering apparatus, a steering wheel, when rotating on, finally comes into contact with a steering stopper with the result that further rotations are hindered. If a driver energetically turns the steering wheel, however, an excessive impingement upon the steering stopper might occur. In such a case, an impact as strong as approximately 98.1 kN (10 tons) at the maximum might occur on the rack shaft.
Such a strong impact, when occurred, might exceed an initial axial force of the presser member for preventing the bearing from coming off the ball screw nut, and, when traveling oscillations are added, the presser member might slacken.
If the presser member is screwed to the ball screw nut with a strong torque in order to increase the initial axial force for preventing the presser member from slackening, a ball rolling path formed in the ball screw nut might deform large enough to cause a decline of function of the ball screw nut.
Further, the electrically driven power steering apparatus using the electric motor has been used in terms of saving a fuel cost over the recent years. In the electrically driven power steering apparatus, the electric motor supplied with the electric power from a battery supplies an assistive steering force, and therefore the power may not be taken out of an internal combustion engine directly. Hence, the fuel cost can be saved.
By the way, for instance, in the rack-and-pinion type electrically driven power steering apparatus, if a traveling wheel collides with a paved step of sidewalk during traveling of the vehicle, a large impact might be transferred to the rack shaft from tie rods. Such an impact is transferred tracing back a steering force transfer route and might exert a strong stress on the respective members. While on the other hand, a sufficient rigidity against such an impact is ensured, a problem is that the respective members increase both in sizes and weights.